With the progress of technology, electronic products demand more and more from power supplies and therefore voltage regulators have attracted more and more attention. Conventional voltage regulators comprise switching regulators and low dropout (LDO) regulators, both functioning for converting input voltages to stable output voltages. However, in some certain situations, such voltage regulators may generate huge currents to damage components thereof or in their load circuits and thus an over current protection (OCP) mechanism is employed to prevent the voltage regulators from outputting excessive currents. Many prior arts, for example U.S. Pat. No. 6,091,610 to Garcia et al., have been proposed with OCP methods.
FIG. 1 is a simplified circuit diagram of a conventional boost switching regulator 100, in which an inductor L and a diode D1 are serially connected between a power input receiving an input voltage Vin and a power output providing an output voltage Vout, a switch 102 is connected between a switch node LX and ground GND, a pulse width modulation (PWM) controller 104 provides a PWM signal to switch the switch 102 so as to convert the input voltage Vin to the output voltage Vout, voltage divider resistors R1 and R2 are serially connected between the power output Vout and ground GND for dividing the output voltage Vout to generate a feedback voltage VFB, the PWM controller 104 determines a duty cycle of the switch 102 according to the feedback voltage VFB and a reference voltage Vref, and an OCP controller 106 monitors the input current Iin flowing through the inductor L by monitoring the feedback voltage VFB and the reference voltage Vref. When the inductor current Iin increases to reach a threshold level, the OCP controller 106 triggers an OCP signal to the PWM controller 104 so as to alter the duty cycle of the switch 102 and thereby restrict the inductor current Iin below the threshold level.
FIG. 2 is a simplified circuit diagram of a conventional LDO regulator 200, in which a switch 202 is connected between a power input receiving an input voltage Vin and a power output providing an output voltage Vout, voltage divider resistors R1 and R2 are serially connected between the power output Vout and ground GND for dividing the output voltage Vout to generate a feedback voltage VFB, an LDO controller 204 determines the input current Iin flowing through the switch 202 according to the feedback voltage VFB and a reference voltage Vref, and an OCP controller 206 monitors the input current Iin by monitoring the feedback voltage VFB and the reference voltage Vref. When the input current Iin increases to reach a threshold level, the OCP controller 206 triggers an OCP signal to the LDO controller 204 to restrict the input current Iin.
However, in a portable electronic product, the input voltage Vin is provided by a battery and gradually decreases with the duration of use. According to a principle that an input power shall be equal to an output power, it can be derived thatVin×Iin=Vout×Iout,  [Eq-1]where Iout is the output current of the switching regulator 100 or the LDO regulator 200. Since the output voltage Vout and the output current Iout are designed to maintain at constant levels, as shown in FIG. 3, when the input voltage Vin decreases as indicated by the waveform 300, the input current Iin will increases in response thereto as indicated by the waveform 304. If the input voltage Vin decreases to be excessively low, the input current Iin will increase to reach the OCP threshold level IOCP and may trigger the over current protection accordingly. To a battery system, such situation will result in the input voltage Vin decreasing more steeply. When the input voltage Vin decreases to be lower than a under-voltage lockout threshold level UVLO, it means that the battery is no longer capable of providing sufficient voltage Vin, and thus the system will disable the controllers 104 and 204, as indicated by the waveform 302.
In a photoflash capacitor charger with a part no. LD7266 provided by Leadtrend Technology Corporation, when the battery voltage is lower than a preset value, the duty cycle of the power switch will be adjusted to control the output current so as to extend the battery lifetime. However, such approach for controlling the output current is not suitable to voltage regulators because the feedback loop in a voltage regulator operates to control the feedback voltage VFB close to the reference voltage Vref. In normal operations, the input current Iin of a voltage regulator is not equal to the OCP threshold level IOCP and furthermore, in the switching regulator 100 of FIG. 1, if the inductance of the inductor L is changed, the input voltage Vin to trigger the over current protection will change correspondingly and this changes the slope of the output voltage Vout. FIG. 4 is a diagram showing the output voltage Vout vs. the input current Iin in the switching regulator 100 of FIG. 1 under different inductances of the inductor L, in which waveform 400 represents the threshold level IOCP, waveform 402 represents the input current Iin when the inductance is LB, waveform 404 represents the input current Iin when the inductance is LA, waveform 406 represents the output voltage Vout when the inductance is LA, and waveform 408 represents the output voltage Vout when the inductance is LB, where LA>LB. As shown in FIG. 4, the performance of the output voltage Vout will be different if the switching regulator 100 employs different inductor L.
Therefore, it is desired a circuit and method for a voltage regulator for longer battery lifetime provided to the voltage regulator.